The present invention relates to an orthogonal modulator for use in a digital mobile communication device, such as a portable telephone, and, more particularly, to an orthogonal modulator less susceptible to the influence of a carrier leak.
FIG. 1 is a schematic block diagram of a conventional orthogonal modulator 11. The orthogonal modulator 11 includes a frequency multiplier 12, a phase shifter 13, modulation mixers 14 and 15, and an adder 16, all formed on a single semiconductor substrate.
The orthogonal modulator 11 is connected to a transformer 17 called a balloon to suppress the occurrence of a carrier leak. The transformer 17 receives a carrier signal LOin and supplies a carrier signal LO in phase with the carrier signal LOin and a carrier signal LOx of the opposite phase to that of the carrier signal LOin, to the orthogonal modulator 11.
The frequency multiplier 12 multiplies the frequencies of the complementary carrier signals LO and LOx by two and supplies frequency-doubled carrier signals 2LO and 2LOx to the phase shifter 13. The phase shifter 13 divides the frequencies of the frequency-doubled carrier signals 2LO and 2LOx by two to generate four carrier signals LO0, LO90, LO180 and LO270 whose phases are shifted from one another by 90 degrees. The carrier signals LO0 and LO180 are complementary to each other, and the carrier signals LO90 and LO270 complementary to each other.
The modulation mixer 14 multiplies a baseband signal I or Ix by the carrier signal LO0 or LO180 to produce a first modulation signal VI. The modulation mixer 15 multiplies a baseband signal Q or Qx by the carrier signal LO90 or LO270 to produce a second modulation signal V2. The adder 16 adds the first and second modulation signals V1 and V2 together and outputs an output signal RFout.
A double mode phenomenon, which is caused by the carrier leak, makes the modulating operation of the orthogonal modulator 11 to be unstable. The double mode phenomenon includes a good mode which indicates the spectrum of the output signal RFout as shown in FIG. 3A and a bad mode indicating the spectrum of the output signal RFout as shown in FIG. 3B. The mode (i.e., the good mode or the bad mode) is determined by the timing of powering on a portable device and the carrier signals LO and LOx. Referring to FIGS. 3A and 3B, a component CL appears at the frequencies of the carrier signals LO and LOx, and a component Pout appears at positions shifted on the high-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx. A component IR appears at positions shifted on the low-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx.
It is believed that the double mode phenomenon may attributed to the following two factors. The first factor is phase differences between the carrier signal LO or LOx and the carrier signals LO0, LO90, LO180 and LO270. The phase shifter 13 operates to make the rising edge of the frequency-doubled carrier signal 2LO match with the rising edge of the carrier signal LO0. However, the rising edge of the carrier signal LO0 coincides with the rising edge of the carrier signal LO in some cases as shown in FIG. 2A and coincides with the falling edge of the carrier signal LO in other cases as shown in FIG. 2B. That is, the phase shifter 13 generates a carrier signal LO0 having a phase difference of 0 degrees to the carrier signal LO or a carrier signal LO0 having a phase difference of 180 degrees to the carrier signal LO.
The second factor is the generation of a direct current (DC) component on the output signal RFout. As the carrier signal LO or the output signal RFout has a high frequency, it easily leaks in space. As this leaked carrier enters the modulation mixers 14 and 15 through their input terminals, the DC component appears.
More specifically, the output signals Vout (output signals V1 and V2) of the modulation mixers 14 and 15 in an ideal state where the carrier signal LO does not leak are expressed by the following equation (1).                                                                         V                ⁢                                  xe2x80x83                                ⁢                out                            =                              xe2x80x83                            ⁢                                                cos                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        LO                                            ⁢                      t                                        )                                                  xc3x97                                  cos                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        BB                                            ⁢                      t                                        )                                                                                                                          =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  {                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                    f                              LO                                                        +                                                          f                              BB                                                                                )                                                                    ⁢                      t                                        +                                          cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                    f                              LO                                                        -                                                          f                              BB                                                                                )                                                                    ⁢                      t                                                        }                                                                                        (        1        )            
When the leaked carrier signal LO is input to the input terminal for the baseband signal I, Ix, Q or Qx, the output signals Vout of the modulation mixers 14 and 15 are given by the following equation (2).                                                                         V                ⁢                                  xe2x80x83                                ⁢                out                            =                                                cos                  ⁡                                      (                                                                  2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          LO                                                ⁢                        t                                            +                                              φ                        1                                                              )                                                  xc3x97                                  cos                  ⁡                                      (                                                                  2                        ⁢                                                  xe2x80x83                                                ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          LO                                                ⁢                        t                                            +                                              φ                        2                                                              )                                                                                                                          =                                                1                  2                                ⁢                                  {                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                    2                              ⁢                                                              f                                LO                                                                                      +                                                          φ                              1                                                        +                                                          φ                              2                                                                                )                                                                    ⁢                      t                                        +                                          cos                      ⁡                                              (                                                                              φ                            1                                                    -                                                      φ                            2                                                                          )                                                                              }                                                                                        (        2        )            
where xcfx861 is the input phase of the original carrier signal LO and xcfx862 is the leak phase (phase delay) of the leaked carrier signal LO, with the baseband signals ignored for the sake of convenience. The leak phase xcfx862 is nearly constant, and the input phase xcfx861 is 0 degree or 180 degrees (90 degrees or 270 degrees). Thus, the second term in the equation (2) that represents the DC component has two values. The two values for the second term causes the double mode phenomenon. Note that the first term in the equation (2) is hardly affected by the input phase xcfx861 and the leak phase xcfx862 because 2fLO is sufficiently larger than those phases.
Referring now to FIG. 4, an improved orthogonal modulator 21 suppresses the doube mode phenomenon is shown.
The improved orthogonal modulator 21 has two xc2xd frequency dividers 22 and 23, each of which includes a flip-flop type phase shifter.
The first xc2xd frequency divider 22 frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals of phases different by 90 degrees from each other. The second xc2xd frequency divider 23 frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals. A first modulation mixer 24 multiplies the first baseband signal I by the frequency-divided signal from the first xc2xd frequency divider 22. A second modulation mixer 25 multiplies the second baseband signal Q by the frequency-divided signal from the first xc2xd frequency divider 22.
An adder 26 combines the output signals, Iout and Qout, from the first and second modulation mixers 24 and 25, amplifies the resultant signal and outputs the amplified signal Sout. A frequency multiplier 27 multiplies the amplified signal Sout from the adder 26 by the output signal of the second xc2xd frequency divider 23 to yield an output signal RFout.
The output signals Iout and Qout of the first and second modulation mixers 24 and 25, the output signal Sout of the adder 26 and the output signal Vout of the frequency multiplier 27 are given by the following equations (3) to (6)                                                                         I                ⁢                                  xe2x80x83                                ⁢                out                            =                                                cos                  ⁡                                      (                                          2                      ⁢                                              π                        ·                                                                              f                            LO                                                    /                          2                                                ·                        t                                                              )                                                  xc3x97                                  cos                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        BB                                            ⁢                      t                                        )                                                                                                                          =                                                1                  2                                ⁢                                  {                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                                                    f                                LO                                                            /                              2                                                        +                                                          f                              BB                                                                                )                                                                    ⁢                      t                                        +                                          cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                                                    f                                LO                                                            /                              2                                                        -                                                          f                              BB                                                                                )                                                                    ⁢                      t                                                        }                                                                                        (        3        )                                                                                    Q                ⁢                                  xe2x80x83                                ⁢                out                            =                                                cos                  ⁡                                      (                                                                  2                        ⁢                                                  π                          ·                                                                                    f                              LO                                                        /                            2                                                    ·                          t                                                                    -                                              90                        ⁢                        xc2x0                                                              )                                                  xc3x97                                  cos                  ⁡                                      (                                                                  2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          BB                                                ⁢                        t                                            +                                              90                        ⁢                        xc2x0                                                              )                                                                                                                          =                                                1                  2                                ⁢                                  {                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                                                    f                                LO                                                            /                              2                                                        +                                                          f                              BB                                                                                )                                                                    ⁢                      t                                        -                                          cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                                                    f                                LO                                                            /                              2                                                        -                                                          f                              BB                                                                                )                                                                    ⁢                      t                                                        }                                                                                        (        4        )                                                                                    S                ⁢                                  xe2x80x83                                ⁢                out                            =                                                I                  ⁢                                      xe2x80x83                                    ⁢                  out                                +                                  Q                  ⁢                                      xe2x80x83                                    ⁢                  out                                                                                                        =                              cos                ⁢                                  xe2x80x83                                ⁢                2                ⁢                                  π                  ⁡                                      (                                                                                            f                          LO                                                /                        2                                            +                                              f                        BB                                                              )                                                  ⁢                t                                                                        (        5        )                                                                                    V                ⁢                                  xe2x80x83                                ⁢                out                            =                              S                ⁢                                  xe2x80x83                                ⁢                out                xc3x97                                  cos                  ⁡                                      (                                          2                      ⁢                                              π                        ·                                                                              f                            LO                                                    /                          2                                                ·                        t                                                              )                                                                                                                          =                              cos                ⁢                                  xe2x80x83                                ⁢                2                ⁢                                  π                  ⁡                                      (                                                                                            f                          LO                                                /                        2                                            +                                              f                        BB                                                              )                                                  ⁢                t                xc3x97                                  cos                  ⁡                                      (                                          2                      ⁢                                              π                        ·                                                                              f                            LO                                                    /                          2                                                ·                        t                                                              )                                                                                                                          =                                                1                  2                                ⁢                                  {                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                                              π                        ⁡                                                  (                                                                                    f                              LO                                                        +                                                          f                              BB                                                                                )                                                                    ⁢                      t                                        +                                          cos                      ⁢                                              xe2x80x83                                            ⁢                      2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        BB                                            ⁢                      t                                                        }                                                                                        (        6        )            
It is apparent from those equations (3) to (6) that the influence of the carrier leak is repressed. Further, the provision of the frequency multiplier 27 on the output terminal side allows the frequencies of the output signals Iout and Qout of the modulation mixers 24 and 25 to be half the frequencies of the output signals V1 and V2 of the modulation mixers 14 and 15 in FIG. 1. This ensures stable operation, and lower current consumption, of the orthogonal modulator 21.
The first and second xc2xd frequency dividers 22 and 23 have relative large circuit scales. Accordingly, the chip size of the orthogonal modulator 21 becomes inevitably larger, which stands in the way of size reduction of portable devices. Further, the increased chip size increases the cost of the orthogonal modulator 21 and eventually the costs of portable devices using or incorporating the modulator 21.
Accordingly, it is an objective of the present invention to provide a modulation mixer and an orthogonal modulator which reduce the influence of a carrier leak at a low cost.
Briefly stated, the present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a transistor receiving the baseband signal, and an element, connected to the transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the transistor.
The present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a transistor receiving the baseband signal, and an element, connected to the transistor, for reducing a frequency characteristic of the transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the transistor.
The present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a first transistor receiving the baseband signal, and a second transistor receiving the carrier signal. The first transistor has a larger size than the second transistor to reduce a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the first transistor.
The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal having a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a first element, connected to the first transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a second transistor receiving the second baseband signal, and a second element, connected to the second transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the second transistor.
The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier for receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal having a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal, and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a first element, connected to the first transistor, for reducing a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a second transistor receiving the second baseband signal, and a second element, connected to the second transistor, for reducing a frequency characteristic of the second transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the second transistor.
The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier for receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal, and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal, and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a second transistor receiving the in-phase carrier signal. The first transistor has a larger size than the second transistor to reduce a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a third transistor receiving the second baseband signal, and a fourth transistor receiving the orthogonal carrier signal. The third transistor has a larger size than the fourth transistor to reduce a frequency characteristic of the third transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the third transistor.
The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a phase shifter for receiving a carrier signal having a predetermined frequency and dividing the frequency of the carrier signal by two to generate an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. A first frequency multiplier is connected to the first modulation mixer and the phase shifter, receives the first modulation signal and one of the in-phase carrier signal and the orthogonal carrier signal and multiplies the first modulation signal by the one of the in-phase carrier signal and the orthogonal carrier signal to produce a third modulation signal. A second frequency multiplier is connected to the second modulation mixer and the phase shifter, receives the second modulation signal and one of the in-phase carrier signal and the orthogonal carrier signal and multiplies the second modulation signal by the one of the in-phase carrier signal and the orthogonal carrier signal to produce a fourth modulation signal. An adder is connected to the first and second frequency multipliers, receives the third and fourth modulation signals and adds the third and fourth modulation signals to produce the modulation signal of the orthogonal modulator.
The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a phase shifter for receiving a carrier signal having a predetermined frequency and dividing the frequency of the carrier signal by two to generate an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a second baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the second baseband signal to produce a second modulation signal. A first frequency multiplier is connected to the first modulation mixer and the phase shifter, receives the first modulation signal and the in-phase carrier signal and multiplies the first modulation signal by the in-phase carrier signal to produce a third modulation signal. A second frequency multiplier is connected to the second modulation mixer and the phase shifter, receives the second modulation signal and the orthogonal carrier signal and multiplies the second modulation signal by the orthogonal carrier signal to produce a fourth modulation signal. An adder is connected to the first and second frequency multipliers, receives the third and fourth modulation signals and adds the third and fourth modulation signals to produce the modulation signal of the orthogonal modulator.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.